We propose to examine the relationship between the binding of agonists to Alpha1 and Beta2-adrenergic receptors and subsequent contraction or relaxation of smooth muscle. Alpha1 and Beta2 receptors in rat vas deferens and caudal artery will be labelled with 125IBE 2254 and 125I-iodocyanopindolol. KD values for antagonist inhibition of radioligand binding will be correlated with pA2 values for inhibition of agonist induced contraction or relaxation. Comparison of KD values for agonist inhibition of radioligand binding and Kact values for contraction or relaxation will give an estimate of the occupancy-response relationship ("spare receptor" capacity) of each tissue. Occupancy-response relationships will be determined directly by progressive inactivation of Alpha1 and Beta2 receptors with the nonequilibrium Alpha- and Beta-adrenergic blocking agents phenoxybenzamine and FM 24. Comparing the degree of receptor inactivation and alterations in contractile responsiveness will determine the "spare receptor" capacity of each tissue. Occupancy-response relationships will also be examined after various treatments designed to increase or decrease receptor density. These treatments will include DOCA-salt hypertension, cold stress, and chronic drug treatment with adrenergic and nonadrenergic antihypertensive drugs such as prazosin, clonidine, propranolol, guanethidine, nifedipine and verapamil. To determine whether neuronally released NE protects a different receptor pool from exogenously added NE, selective "protection" by field stimulation of norepinephrine release will be compared with addition of exogenous norepinephrine during receptor inactivation. Possible interactions between Alpha1 and Beta2-adrenergic receptors will also be examined by inactivating a single receptor type and looking at the properties and responsiveness of the other receptor type. These experiments should increase our understanding of the relationship between adrenergic receptor occupancy and the contractile responses of smooth muscle, and the adaptive mechanisms involved in changes in tissue sensitivity to receptor stimulation. This should cotribute to a further understanding of pathophysiological alterations in adrenergic receptor sensitivity such as are seen following chronic drug administration or diseases such as hypertension and asthma.